Investment casting

ABSTRACT

Methods are provided for producing investment cast articles, such as orthopedic implants, or portions thereof, having at least a partially textured surface that is formed during casting of the article. In an exemplary method, a textured metal casting is produced by creating a heat destructible pattern and spraying the pattern with a texturing material to cause the texturing material to form a textured surface on at least a portion of the pattern. In another embodiment, a textured template is pressed against a heat softenable pattern to provide a textured pattern. With respect to each of these methods, a shell is the created around the textured pattern to form a mold, and the pattern is removed from the shell. Molten metal is introduced into the mold and allowed to harden, after which the mold is removed.

FIELD OF THE INVENTION

The present invention relates to investment casting, and moreparticularly to the manufacture of investment cast articles, such asorthopedic implants, with a textured surface.

BACKGROUND OF THE INVENTION

Various metal casting processes, such as investment (or "lost wax")casting are well known for the fabrication of metal objects. Thisprocess requires several steps, the first of which is to create orprovide a pattern or shape to be replicated. The pattern, often made ofwax, is used to make a mold that is then used to form cast metalarticles.

Typically, several wax patterns are joined together on a wax "tree" toenable the simultaneous manufacture of several parts. The tree is asolid wax tube that has side walls to which a stem of each wax patternis joined to form a cluster. The wax tree defines what will become agate leading to passages for allowing molten metal to travel through themold to each cluster and part pattern. Once all of the wax patterns arejoined to the wax tree, the cluster is coated with one or more coats ofa refractory by dipping the wax pattern-tree assembly in a ceramicslurry. After the slurry dries, fabrication of a shell or mold iscompleted by heating the slurry coated wax to cure or harden theceramic, and to melt out the solid wax patterns and the wax tree. Moltenmetal is then poured into the shell so that it fills each of thecavities formerly occupied by the wax patterns and the wax tree. Afterthe metal has cooled and hardened, the shell is fractured and removed,and the cast metal parts are severed from the metal tree. The cast partsare then subjected to post-machining, grinding off the gates, beadblasting, and polishing, as required.

With respect to medical implants, such as joint prosthesis components,it has been discovered that texturing or roughening the surface of acast metal implant can improve the interface and fixation between theimplant and the bone, with or without bone cement. The creation of aroughened surface on an implant, whether it was investment cast orforged, is typically one of the last steps in the manufacturing processof the component. Known methods for obtaining a roughened surfaceinclude grit-blasting, grinding, direct machining, laser etching, andsintering of beads to the surface of the implant.

Known surface texturing techniques, however, have severe short-comingswith respect to manufacturing speed, efficiency, and cost, as well asstructural limitations and deficiencies. For example, if the surface ofthe implant is improperly roughened, the near finished implant must bediscarded. Depending on the alloy used to fabricate the implant and thesize of the implant, the creation of unacceptable texturing cansignificantly increase manufacturing cost. The creation of a texturedsurface requires a skilled craftsperson. Therefore, for other thansimple, uniform texturing, precise replication of a particularconfiguration is difficult to achieve in even limited productionquantities. Furthermore, known surface texturing techniques areincapable of creating certain complex shapes and patterns, such asundercutting, which foster bone ingrowth and more secure fixation. Yetanother disadvantage of known techniques for texturing a previously-castimplant is that the application of texturing materials, such as with aplasma-spray process results in an imperfect bond between the sprayed-onmaterial and the implant which can lead to abrasive detachment and weaksubstrate coating interfaces. Similarly, sintering beads on a surfacecauses loss of favorable mechanical properties.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of known surfacetexturing techniques by providing a heat destructible pattern having atextured surface. The textured pattern is used to create an investmentcasting mold which in turn is used to create the cast article. Thistechnique allows very specific and/or complex patterns to be integrallyformed with the implant surface as cast.

In an exemplary method, a textured metal casting, such as an orthopedicimplant or a component thereof, is produced by creating a heatdestructible pattern and spraying the pattern with a texturing materialto cause the texturing material to form a textured surface on at least aportion of the pattern. The texturing material can include molten waxdroplets that adhere to and/or melt at least a portion of the surface ofthe pattern or solid particles that adhere to and/or indent the surfaceof the pattern. The texturing material can be evenly or unevenly sprayedagainst the pattern at a selected temperature and velocity to create adesired texture. A shell is created around the textured pattern to forma mold, and the pattern is removed from the mold. Molten metal isintroduced into the mold and allowed to harden, after which the mold isremoved from the textured casting.

In another embodiment, a textured template is pressed against a heatsoftenable pattern to provide a textured pattern. The textured templatecan be formed by creating a data file that defines a three-dimensionaltextured template; providing the data file to a rapid prototypingmachine; and forming a model representative of the three-dimensionaltextured template with the rapid prototyping machine. A second texturedtemplate can be provided that has a different texture than the firsttextured template.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and the attendantadvantages and features thereof will be more readily understood byreference to the following detailed description when it is considered inconjunction with the accompanying drawings wherein:

FIG. 1 is a simplified illustration of a pattern being sprayed with atexturing material;

FIG. 2 is a sectional view of a portion of a pattern having wax spraytexturing to create positive relief features;

FIG. 3 is a sectional view of a portion of a pattern having wax spraytexturing to create negative relief features;

FIG. 4 is a sectional view of a portion of a pattern having solidparticle texturing; and

FIG. 5 is a schematic view of a pattern being textured with a template.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of a method of making a textured investment castingbegins with the step of creating a custom heat destructible pattern orselecting an existing pattern from stock supplies. Typically, patternsare made of investment casting wax, however, many plastics are alsoacceptable, as are hybrid wax/plastic patterns. As used herein, a"pattern" is full-scale representation or model of any article that iseither machine or hand made. Exemplary patterns include components fororthopedic implants and portions thereof.

After the pattern is selected or created, a heat destructible texturingmaterial is selected for spraying against the pattern. Exemplarytexturing materials include wax droplets or beads that are molten, soft,or hard, and plastic particles or beads. However, any material which maybe applied to the pattern and which will subsequently burn out of a moldcleanly may be used. As used herein, "particle" or "droplet" is intendedto encompass any size or shape configuration obtainable with a giventexturing material, and the particular size and configuration ormorphology of a selected particle or droplet is selected to achieve adesired texture.

Following selection of a texturing material, a spray is created with thetexturing material and it is directed toward the pattern or the patternis placed in the path of the spray. As used herein, a "spray" isintended to encompass everything from a concentrated, narrow stream to adiffuse mist, and "spray" should be broadly construed to mean placingthe texturing material or a portion thereof in motion. The particulardispersion, concentration, temperature, and pressure of the spray aredetermined by the texturing material and the desired texture for thepattern. As illustrated in FIG. 1, molten wax or solid particles 10,such as wax or plastic beads under pressure can be ejected from a nozzle12 to create a spray. A pattern 14 is shown in the path of the spray.

FIG. 2 illustrates a pattern 14 that has been sprayed by molten waxdroplets, wherein the molten wax temperature and spray velocity weresuch that at least some of the molten wax droplets adhered to at least aportion of the surface 16 of the pattern to create surface texturing,roughness, or bas relief. In FIG. 2, the droplets have been deposited toform bumps that can be defined as positive surface relief features, on aportion of the pattern surface 16.

FIG. 3 illustrates a pattern 14 that has been sprayed by molten waxdroplets, wherein the molten wax temperature and spray velocity weresuch that at least some of the molten wax droplets melted away at leasta portion of the surface 16 of the pattern to create surface texturing,roughness, or bas relief. In FIG. 3, the droplets have been createddepressions, voids, or indentations 20 that can be defined as negativesurface relief features, on a portion of the pattern surface 16.

The temperature of the spray of molten wax can be regulated to createpositive and negative surface features in the same surface areas. Also,the spray of texturing material can be directed toward the pattern 16,or a portion thereof, to evenly or unevenly distribute the spray tocreate, respectively, a substantially uniform or irregular pattern.Furthermore, although the bumps 18 and indentions 20 appear rounded inthe illustrations, their shapes can range from circular to linear, andthe bumps and indentations can have angled or convoluted boundaries.

FIG. 4 illustrates a pattern 22 that has been sprayed by solid particles24, wherein the particle temperature, spray velocity, and particlemorphology were such that at least some of the particles adhered to atleast a portion of the surface 26 of the pattern 22 to create surfacetexturing, roughness, or bas relief. The particles 24 form bumps 28 thatcan be defined as positive surface relief features, on a portion of thepattern surface 26. The particles can range from being barely to fullyimpacted in the surface of the pattern. Bumps are also created by thedisplacement of pattern material in response to particle impact.

FIG. 4 also illustrates additional surface texturing, roughness, or basrelief in the form of depressions, voids, or indentations 30 that can bedefined as negative surface relief features. These indentations arecreated by impact of the particles 24 with the surface 26 and thenbouncing off or not adhering to the surface. As with respect to themolten wax spray technique, the temperature of the particulate spray canbe regulated to create positive and negative surface features in thesame surface areas. Similarly, the particulate spray of texturingmaterial can be directed toward the pattern 22, or a portion thereof, toevenly or unevenly distribute the spray to create, respectively, asubstantially uniform or irregular pattern. Furthermore, although thebumps 28 and indentions 30 appear rounded in the illustrations, theirshapes can range from circular to linear, and have angled or convolutedboundaries. If the particles impact the surface at an angle, anunder-cut surface 32 is created.

Yet another technique for creating a textured pattern includes pressinga textured template 34 against at least a portion of a heat softenablepattern 36 to provide a textured pattern, as shown in FIG. 5. Either thetextured template 34, the pattern 36, or both can be heated tofacilitate pattern creation. Alternatively, the template can create atexture on the pattern by pressing the template against the pattern withenough pressure to deform the pattern. This technique is well suited forapplications requiring a precise and/or complicated pattern geometry,and it is particularly well suited for providing a texture that promotesbone ingrowth.

In an exemplary embodiment, the template 34 is a metal object fabricatedusing any known technique for creating a metal object. Alternatively,the template 34 can be fabricated from a plastic or refactory materialthat has a higher melting/distortion point than the wax pattern.However, in other embodiments, the template is created with a rapidprototyping machine. The object created by the rapid prototyping machinecan itself be used as the textured template, or it can be used as amodel for the creation of an investment casting mold.

For example, a textured template can be provided by creating a data filethat defines a three-dimensional textured template, wherein the texturedtemplate includes a texture pattern configured to promote bone ingrowth.The data file is provided to a rapid prototyping machine capable ofcreating a heat destructible model. A heat destructible modelrepresentative of the three-dimensional textured template is thencreated with the rapid prototyping machine. A heat resistant shell isformed around the heat destructible model and heat is applied to theshell to remove the heat destructible model therefrom. Molten metal ispoured into the shell; the molten metal hardens; and the shell isremoved. The thus formed textured template is heat pressed against atleast a portion of the pattern to provide a textured pattern. A secondtextured template having a different texture than the first texturedtemplate can be provided and pressed on at least a portion of thepattern to provide a pattern with different textures on differentregions thereof.

Having been imparted with a surface texture, the pattern, created bywhatever method, is used to create a mold. In an exemplary method, thepattern is coated with particles of a refractory material by spraying ordipping to create a shell around the textured pattern. The texturedpattern is removed from the shell by the application of heat whichcauses the pattern, and any blast media impacted thereon, to melt, burn,or vaporize, allowing it to be drained or exhausted from an opening inthe shell. Heating, either in this or a subsequent step, cures therefractory material to complete the mold making process.

Subsequent casting steps are not discussed in great detail, as they arewell known to those skilled in the art of metal casting. These stepsinclude introducing molten metal, such as Cr--Co--Mo alloy (ASTM F75)through one or more gates in the mold. The mold and the metal areallowed to cool, and the mold is removed from the hardened metal. Thetextured implant can then be subjected to post-machining, grinding offof gates, bead blasting, and polishing, as required.

Each of the above techniques provides different manufacturingadvantages. However, should a problem be encountered in the texturingsteps, the cost associated with scrap is greatly reduced as onlyinexpensive wax is sacrificed rather than a nearly finished product.Accidentally roughened surfaces are easily smoothed by heating the wax.

Although the invention has been shown and described with respect toexemplary embodiments thereof, various other changes, omissions andadditions in form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method of making a textured metal castingcomprising the steps of:creating a heat softenable pattern; creating atextured template by creating a data file that defines athree-dimensional textured template, providing the data file to a rapidprototyping machine, and forming a model representative of thethree-dimensional textured template with the rapid prototyping machine;pressing the textured template against at least a portion of the patternto provide a textured pattern; creating a shell around the texturedpattern; removing the textured pattern from the shell; introducingmolten metal into the shell; allowing the molten metal to harden;removing the shell from the hardened metal.
 2. The method of claim 1,wherein the pattern includes a representation of at least a portion ofan orthopedic implant.
 3. The method of claim 2, wherein the texturedpattern is configured to promote bone ingrowth.
 4. The method of claim2, wherein the heat softenable material includes wax.
 5. The method ofclaim 2, wherein the heat softenable material includes plastic.
 6. Themethod of claim 1, further comprising the step of heating the templateto a temperature sufficient to at least soften the pattern.
 7. Themethod of claim 1, further comprising the step of heating the pattern toa temperature sufficient to soften at least a surface portion of thepattern.
 8. The method of claim 1, wherein the template includes metal.9. The method of claim 1, wherein the template includes a plasticmaterial.
 10. The method of claim 1, further comprising the stepsof:creating a second textured template, wherein the second texturedtemplate has a different texture than the first textured template; andpressing the second textured template on at least a portion of thepattern.
 11. A method of making a textured metal casting comprising thesteps of:creating a heat softenable pattern representative of at least aportion of an orthopedic implant; creating a data file that defines athree-dimensional textured template; providing the data file to a rapidprototyping machine capable of creating a model consisting of arefractory material; forming a ceramic model representative of thethree-dimensional textured template with the rapid prototyping machine;pressing the ceramic model against at least a portion of the pattern toprovide a textured pattern; creating a shell around the texturedpattern; removing the textured pattern from the shell; introducingmolten metal into the shell; allowing the molten metal to harden; andremoving the shell from the hardened metal.
 12. A method of making atextured metal casting comprising the steps of:creating heat softenablepattern that is representative of at least a portion of an orthopedicimplant; creating a textured template bycreating a data file thatdefines a three-dimensional textured template, wherein the texturedtemplate includes a texture pattern configured to promote bone ingrowth;providing the data file to a rapid prototyping machine capable ofcreating a heat destructible model; forming a heat destructible modelrepresentative of the three-dimensional textured template with the rapidprototyping machine; creating a heat resistant shell around the heatdestructible model; applying heat to the heat resistant shell to removethe heat destructible model therefrom; introducing molten metal into theshell; allowing the molten metal to harden to form the texturedtemplate; and removing the textured template from the shell; heatpressing the textured template against at least a portion of the patternto provide a textured pattern; creating a shell around the texturedpattern; removing the textured pattern from the shell; introducingmolten metal into the shell; allowing the molten metal to harden to formthe textured metal casting; and removing the shell from the texturedmetal casing.
 13. The method of claim 12, further comprising the stepsof:creating a second textured template, wherein the second texturedtemplate has a different texture than the first textured template; andpressing the second textured template on at least a portion of thepattern.